A vehicle that has become stuck in a depression-like hollow with spinning wheels, so that the vehicle cannot be driven directly out of the depression because the subsoil friction is insufficient or the subsoil is too yielding, can often be freed from that situation by a so-termed rocking-free maneuver. One possibility for emerging from the depression without outside help consists in rocking the vehicle by driving it cyclically forward and in reverse. In physical terms the forward and backward driving or forward and backward rolling vehicle constitutes an oscillating system which can be energized by a suitable input of energy until the kinetic energy suffices to overcome the edge of the depression.
In vehicles with a manual transmission, the driver can carry out a rocking-free process of that kind by rhythmic actuation of the starting clutch and the accelerator pedal and by repeatedly changing between forward and reverse driving, in order successively to rock the vehicle free.
In vehicles with an automated or automatic transmission automatic rocking-free modes are known, which the driver can switch into or which are automatically engaged by virtue of an automatic rocking-free recognition function. The rocking-free process itself then takes place automatically such that a periodically increasing and decreasing drive torque in the forward and/or the reverse direction is produced. The drive torque can for example be produced by a combustion engine, while a gear of the transmission is automatically engaged or disengaged or while a shift automatically takes place repeatedly between a forward gear and a reverse gear. In the case of a hybrid or electric drive, the drive direction can be changed almost without any delay by alternating the direction of current flow in an electric drive motor.
DE 10 2010 043 250 A1 describes a method for the automatic rocking-free of a motor vehicle having an automatic or automated transmission. A rocking-free function is started if a rocking-free situation is recognized automatically for the motor vehicle or if the driver specifies the same by actuating an operating element, and the function is automatically terminated when the motor vehicle reaches a travel-path dependent or a time-dependent limit. Starting from a position with a stationary wheel in a depression from which the motor vehicle has to be moved clear, to rock it free a drive torque is first built up which acts in a desired travel direction determined by the gear engaged until the motor vehicle moving in the desired travel direction reaches a forward travel direction reversal point of the depression. This reversal point is recognized in that the speed of a non-driven wheel falls to zero or the movement direction of the motor vehicle is reversed. The drive torque is then reduced until the motor vehicle, now moving in a direction opposite to the desired travel direction, reaches a rearward travel direction reversal point, and is then built up again. The process is repeated cyclically, whereby the locations of the reversal points are continually adapted and successively migrate out of the depression until the motor vehicle reaches a forward or rear limit point which marks the end of the depression. The limit points are defined by an admissible travel path from the starting position in the relevant travel direction or by an admissible time during which the drive torque is applied, and are continually adapted.
During rocking free, in addition to the alternate build-up and reduction of the drive torque, when the drive torque is reduced the transmission can be shifted to neutral so that in the neutral transmission position the motor vehicle rolls passively backward, or a shift between a forward and a reverse gear can take place so that an active drive torque acts in both rocking directions.
If an electric motor drive is available, then to change between the drive torque acting in the desired travel direction and the drive torque acting in the opposite direction there is no need for a gear change, since the torque direction can be changed by means of the electric drive.
The rocking free as such is carried out fully automatically. However, the driver can have the option to influence the rocking-free function actively by actuating the accelerator pedal, since the function is activated or deactivated depending on a minimum actuation, i.e. a minimum deflection angle of the accelerator pedal and/or since, depending on the acceleration pedal actuation, in each case a maximum drive torque is determined.
DE 10 2009 036 058 A1 describes a method for rocking a motor vehicle free, in which a dual-clutch transmission has a rocking-free mode such that in a first partial transmission a forward gear and in a second partial transmission a reverse gear are engaged at the same time, and the drive input is automatically switched between the partial transmissions. The rocking-free mode is automatically activated if, within a short time, the driver shifts the transmission repeatedly between forward driving, reversing and neutral. Or else the driver engages the rocking-free mode manually if he recognizes the need for this or when a display recommends this to him. The vehicle is only operated in the rocking-free mode when the driver actuates the accelerator pedal. Furthermore, a distance sensor system ensures that the rocking-free mode is automatically terminated if a collision with an obstacle is possible if rocking-free continues.
In the case of a vehicle with an automated transmission or with an automatic transmission, if the driver does not want fully automatic rocking free, or if no such fully automatic function is available, or if an existing rocking-free function is to be influenced actively by the driver, then a rocking-free function can be carried out and its sequence controlled by alternate actuation and release of the accelerator pedal. A repeated change of the position of a selector lever between its positions for forward drive, reverse and neutral is also possible. For the driver, however, it is difficult to act in phase during rocking free, i.e. to recognize in each case the most favorable time-point for actuating and releasing the accelerator pedal. In addition, owing to the driver's reaction time and the electronic transmission time of the accelerator pedal information to the transmission or clutch control unit and the time taken to convert that information to a corresponding change of a drive torque or a clutch torque, kinetic energy can be lost or dissipated so that the rocking-free process is unnecessarily delayed or even unsuccessful.